1. Field of the Invention
This invention relates to a torsion-axle type rear suspension for a vehicle.
2. Description of the Prior Art
There has been known a torsion-axle type rear suspension as disclosed in, for instance, Japanese Unexamined Utility Model publication No. 58(1983)-58906. As shown in prior art FIGS. 21 and 22 of the present application, the torsion-axle type rear suspension generally comprises left and right wheel supports 2a and 2b for supporting left and right rear wheels 1a and 1b for rotation, a cross member 3 which extends in the transverse direction of the vehicle body and is connected to the wheel supports 2a and 2b at opposite ends, left and right trailing arms 5a and 5b which are fixed to the cross member 3 at the rear ends and connected to the vehicle body 4 at the front ends by way of elastic members 6a and 6b for vertical swinging motion and longitudinal displacement and a lateral rod 8 which is connected to the cross member 3 at one end by way of one elastic member 7 and to the vehicle body 4 at the other end by way of another elastic member 7. The lower ends of shock absorbers 9 which are connected to the vehicle body at the upper ends are connected to the respective junctions of the trailing arms 5a and 5b to the cross member 3. A torsion bar 10 extends inside the cross member 3.
It is generally preferred in order to improve running performance of the vehicle that the toe of the rear wheels in addition to turn of the front wheels in response to turn of the steering wheel. For example, when the steering wheel is turned by a slight angle while the vehicle runs straight, it is preferred that the outer (with respect to the turning direction) rear wheel be caused to toe in to generate understeer tendency, thereby improving running stability of the vehicle. On the other hand, during hard cornering, it is preferred that the outer rear wheel be caused to toe out to weaken strong understeer tendency especially in the case of a front-engine front-drive type vehicle in which understeer tendency is stronger. In another aspect, it is generally preferred that when the steering wheel is turned while the vehicle runs at a low speed, the outer rear wheel be caused to toe out to improve heading performance of the vehicle. On the other hand, during hard cornering or lane change at a high speed, it is preferred that the outer rear wheel be caused to toe in to ensure running stability.
Such behavior of the outer rear wheel can be obtained by controlling the toe of the rear wheel according to the side force that is generated by change of centripetal acceleration and acts on the rear wheel when the steering wheel is turned. However, in the case of the conventional torsion-axle type rear suspension, when side force acts on the rear wheel, the rear wheel is merely caused to toe in or toe out depending on whether the lateral rod is mounted on the vehicle body on the front side or the rear side of the line of action of the side force. That is, when a side force G acts on the rear wheels 1a and 1b in FIG. 22, a rightward rotating moment is generated since the lateral rod 8 is mounted on the vehicle body 4 on the rear side of the line of action of the side force G, and accordingly, the right trailing arm 5b is displaced rearward under deformation of the elastic member 6b while the left trailing arm 5a is displaced forward under deformation of the elastic member 6a, whereby the rear wheels 1a and 1b are turned rightward and the left rear wheel 1a toes in. Though the elastic members 7 on the ends of the lateral rod 8 are also deformed by the side force G, this deformation only displaces the cross member in the transverse direction of the vehicle body and does not affect the toe of the left rear wheel 1a since the trailing arms 5a and 5b are parallel to each other. When the lateral rod 8 is on the front side of the line of action of the side force G, the left rear wheel 1a is caused to toe out as can be understood from the description above. At any rate, the rear wheel only toes out or in when the side force G acts thereon and increase of the side force G with increase in the centripetal acceleration only increases the degree of the toe-in or toe-out.